Hi Malcom,I found the photo confusing too. I'm guessing it's the roof over an existing staircase. If you were going to make a ceiling you wouldn't do it like that so likely it is the supporting structure for a staircase above. Back then I imagine the house might have been a bit defensive and the lower level staircase might have replaced the upper level one when there was less need for a defensive layout.
What a fabulous house! It looks like there are three distinct stone types/building styles in that wall. It wouldn't be surprising if the house has been extensively rebuilt in the past and the layout changed at the same time. Chimneys for example are a relatively modern invention that might have been added after the house was first built.
It would be fun to see more of the house.
I have a very cheap USB inspection camera that cost less than £10 (ebay/amazon). At only 8mm diameter, it doesn't need huge hole to fit through. The distance over which an image is "in focus" is surprisingly quite large - At a guess, from under 500mm to over 3m. Put the illumination LEDs in the end are pretty pathetic, so it needs additional lighting when used in dark areas. Still a handy tool to have in the arsenal.If you have access to an adjacent room and a drill you could poke an articulating inspection camera through and have a look. The camera might be expensive as most are just to look close up. Stronger light, longer focal distance, and articulation once it's through the hole will make it more pricey.